The ability of antisera from animals hyperimmunized against rough gram-negative bacteria to passively project against purified endotoxins derived from heterologous gram-negative bacteria has been quantitated recently and shown to be existant but of a low order of magnitude. The ability of such antisera to protect against active gram-negative bacterial sepsis will now be explored. Specifically, the ability of sera from animals hyperimmunized with rough (Re) mutants of a variety of gram-negative microbes will be tested for their ability to transfer protection against challenge with heterologous gram-negative bacteria. The extent of such protection will be quantitated and compared with "O" specific antibody protection. The ability of the antisera to rough bacteria to protect when administered after the infection is in progress will also be carefully quantitated to obtain critical information on its possible therapeutic (as contrasted to its prophylactic) potential. The initial phases of these studies are to be conducted in experimental animal models.